The present invention relates to a process and to an apparatus for regulating the impact of a light beam, particularly a monochromatic light beam on a target and which is emitted by a source and particularly a laser source.
Particularly in laser spectrometry, this process and this apparatus make it possible to accurately regulate the position and dimension of the impact of a monochromatic laser beam on a target. It also makes it possible to maintain the impact of the laser beam on the target in a fixed position in space.
It is known that in laser spectrometry, it is necessary to accurately determine the position on the target and the dimension of the impact of a laser beam bombarding the said target in order to mark the analysed point and control the dimensions of the volume of material vapourized by this bombardment and thus control the emission of the particles resulting from this impact. In addition, it must be possible to carry out this regulation for several wavelengths, because the results of the spectrometric analysis resulting from the impact of the laser beam on the target are linked with the wavelength of the light of the bombardment beam.
In the special case of laser mass spectrometry this regulation must also make it possible for the ion source which extracts the ions transmitted into the spectrometer from the laser plasma to operate continuously under good conditions. This requires an invariable relative position of the laser impact and the ionic optics of the ion source of the spectrometer.
Laser spectrometry consists of producing microplasmas from a solid surface, locally excited by a focused laser light beam. The thus produced microplasmas then are analysed by means of a spectrometer, particularly a mass spectrometer. The laser beam focusing device generally comprises an optical lens with weak magnification, whose focal point position is dependent on the wavelength of the monochromatic laser light beam. Thus, for example said focal length is longer for an infrared laser light (.lambda.=0.6943.mu.) than for an ultraviolet laser light (.lambda.=0.347.mu.). Microplasmas can be produced by focusing laser light either by the incidence of the laser beam on the surface of a target bringing about the emission of a microplasma on the same side of this surface as the laser beam (by reflection) or by incidence of the beam on the surface of a thin target, which brings about the emission of a microplasma on the other face of the target (by transmission). The energy spectrum of the emitted particles is dependent on the position on the focal point of the laser light beam relative to the surface to be investigated. For the emission of microplasmas to be effective by reflection, it is necessary for the focusing point of the laser beam to be slightly in front of the surface and not on it.
It is known to utilize a visible light illumination of the object to carry out the focusing of the focusing means of the laser beam. This focusing is brought about by optical means for which the path of the visible light, used for regulating the focusing is identical to that of the laser beam. This has the serious disadvantage of making uncertain an optimum regulation of the focusing position of the monochromatic laser bean used (e.g. ultraviolet or infrared light). Thus, as has been stated hereinbefore, the focusing position is dependent on the wavelength of the light traversing the focusing means and as a result it is not possible to correctly regulate in white light the focusing position to be obtained in monochromatic laser light. The dimensions of the volume of the material which will be vapourized cannot be predetermined.
No presently known process or apparatus makes it possible to simultaneously solve the problems of precisely locating the point of impact of the laser beam on the surface of the targets under investigation and the problem of regulating the focusing position of the laser beam. Solving these two problems would make it possible to determine beforehand the optimum interaction conditions between the laser beam and the material under investigation.
The only problem which has been solved at present is that of regulating the position of the impact of the laser beam on the target. This can be carried out very accurately when the optical paths of the bombardment laser beam and of the white light beam used in optical impact location means coincide.
Among the presently known devices using this locating method developed by light spectroscopy, one of them comprises a lens for focusing the laser beam which has a double spherical mirror for deviating the bombardment laser beam and the locating white light beam. By means of a telescopic prism system placed on the optical axis of the lens, the laser beam and the white light beam have the same optical axes level with the incidence on the object and as a result the impact point of the laser beam is located in a clearly defined manner. However, as the wavelengths of the laser beam light and white light differ, the device does not make it possible to accurately determine the focusing position of the laser beam and consequently the dimensions of the impact of said beam on the target. Thus, as the focusing distance differs between monochromatic light and white light, the dimensions of the impact obtained in monochromatic light cannot be foreseen. In this device, the target is carried by a support which can only be moved in a plane perpendicular to the direction of the laser beam. Another disadvantage of the known devices is that when the thickness of the target to be analysed varies, the position of the plasma cluster created by laser impact varies. Although this is not prejudicial in light spectroscopy in mass spectroscopy, said position is no longer continuously adapted to the satisfactory operation of the ion source of the mass spectrometer. The diameter of the impact of the beam on the target is predetermined as a function of the wavelength of the laser beam by defocusing and the use of a series of diaphragms having different diameters connected to the focusing lens. This system is not very suitable for mass spectrometry because it is difficult to fit the diaphragms. Thus, the lens is then placed within a vacuum enclosure, which cannot be opened during the experiment. This is very prejudicial when the experiment requires the successive use of monochromatic laser beams of different wavelengths. Moreover, this device does not make it possible to obtain a definite and distinct sighting of the sample during the experiment, because the laser beams and white light beams used for sighting and observation coincide over a large part of their paths.